Say you joined a company relatively early. If you're still around years later by an IPO, you probably liked that early environment where you had low-to-no management overhead, large direct impact, and everybody was personally invested in getting shit done. For most employees that environment was already slipping away pre-IPO, but you know you're close to the finish line and can't get off now. What remains is quickly going to be gone post-IPO.

The early employees get major cash-outs; some probably don't need to work at all anymore, so those that treat their job as a means-to-an-end and not their identity leave immediately. Maybe there's big incentives to stay for a few years, paid out annually, hung over your head so you don't quit immediately. But after a year of more managers and TPS reports and public company corporate governance you get your payout... That annual bonus structure practically forces you to decide if you want to leave now, or signup for a whole 'nother year of this, so a wave of people leave. After a 2nd year of this they're mostly gone to greener pastures.

Less-early employees still get significant cash-outs in a large event like this. Maybe they use it to take some time off, it's been a long few years. Once you're out for a while, why not look at what else is out there on the market. Some of them find out they liked this new big-company stuff and want more of it, so they'll probably stay for years. Others are like the early employees and want the good-old days back, they have to go find it somewhere else.